Incommensurate charge stripe ordering in La_2-xSr _xNiO₄ for x=(0.33,0.30,0.275)

In this paper we report studies of the charge stripe ordering using high resolution x-ray scattering in the nickelate system La_2-xSr _xNiO₄ with doping levels of x=0.33, x=0.30, and x=0.275. The charge stripes for all doping levels were found to be two dimensional in nature with a high degree of correlation in the a-b plane. The in-plane inverse correlation length in the lower doped systems was greater than that in the x=0.33 system and is greatest for x=0.275, consistent with the stripes becoming less correlated as the doping level is decreased from x=0.33. However, the charge ordering in the x=0.33 system was observed to be more two dimensional in nature with a greater inverse correlation length between planes. The interaction between the lattice and charge order was observed to stabilize the charge ordering, even in the absence of spin ordering. In the x=0.30 and x=0.275 systems no long-range charge order could exist without the presence of the magnetic order. In both systems the charge stripes were incommensurate and on heating the incommensurability increased towards the stable ε=0.33 value as the stripes gained thermal energy to overcome the Coulombic repulsion. In all these systems the integrated intensity of the stripe reflections showed a clear difference in behavior from previous neutron studies with an initial increase in the integrated intensity as the temperature was increased, which was not found in the neutron studies.